If you are using HEM:
1) Go to Session, find the hand you want to post. Double click on it. (A replayer will open).
2) In replayer you will find a button called "Hand History". Upper-Left corner. Press the button. (Another window will open).
3) In the window just press 2+2 and just go to the forums and paste it here. That's it.

Other option would be to use for example some converter like this one:
$0.05/$0.10 No Limit Holdem
3 Players
Hand Conversion Powered by weaktight.com

AH! It works. What's confusing is when you push the 2+2 button nothing appears to happen but it automatically pastes the info to your clip board. Then just come here and press paste and it will be pasted into the forum.